1. Field of the Invention
Example embodiments in general relate to providing ergonomically efficient cordless power tools as evidenced by desirable power-to-weight ratios, obtainable in part by reducing weight in one or more constituent weight groups of a given cordless power tool, while maintaining or improving the power output of the tool.
2. Description of Related Art
Users of cordless power tools such as drills, reciprocating saws, circular saws, hammer drills, etc., traditionally sacrifice the enhanced power features of corded tools for the advantages of a cordless environment, i.e., flexibility and portability. While corded power tools may generally offer the user greater power, cordless power tools offer the user ease of use.
A cordless power tool includes a self-contained power source (attached battery pack) and has a reduced power output as compared to a corded tool, due to the limitation on energy density of the cells in the battery pack due to impedance and voltage. Corded power tools thus offer greater power with less weight, as compared to cordless power tool systems. Thus, one problem is that a cordless power tool, in general, cannot closely approximate the performance of a corded power tool. Another problem is that the weight of a cordless power tool for a given power output may be higher and/or substantially higher than its corded counterpart.
From an ergonomic perspective, a way to evaluate tool system performance of a cordless tool is to determine the power-to-weight ratio of a given cordless power tool, and to compare it to the power-to-weight ratio of its corded counterpart, for example. The power-to-weight ratio may be defined as the maximum power output from a motor of a given power tool divided by the total system weight of the tool (system weight=weight of tool and battery pack for cordless power tools; weight of the tool for corded tools). The following provides a general understanding of MWO.
Maximum Watts Out (MWO)
Maximum Watts Out (MWO) generally describes the maximum amount of power out of a power tool system. For example, MWO may be considered to be the maximum power out of a motor of a tool system. Many factors may contribute to the MWO value, the primary factors being source voltage (the source being the battery in a cordless power tool system, the external AC power in a corded tool system), source impedance, motor impedance, current flowing through the system, gear losses and motor efficiency. Secondary factors may affect a power tool system's MWO (such as contact impedance, switch impedance, etc). In some cases, these secondary factors may be considered insignificant contributors as compared to the primary factors.
FIG. 1 is a block diagram of a generic cordless power tool system to describe power losses between the battery source and the motor output. System 100 may include a battery pack 110 which may comprise one or a plurality of cells. For a corded tool, battery 110 would be inapplicable and replaced with an external AC power source, such as a common 15 A, 120 VAC source. Rb 130 represents the internal impedance of the cells comprising the battery 110 (including straps and welds to connect the cells), and Rm 140 represents the internal impedance of motor 120. Motor 120 generally consumes greater current under heavy loads. Switch 150 may be a mechanical or electronic switch (such as a field effect transistor (FET), SCR or other transistor device) that connects the battery 110 to the motor 120.
In FIG. 1, “Vev” represents the electrovoltaic (EV) voltage or the theoretical no-load voltage of the battery 110. “Vbat” represents the actual, measurable voltage of the battery 110 and “Vmotor” denotes the actual, measurable voltage across the motor 120. “Vemf” represents a theoretical voltage presented to the motor 120 for conversion to power.
Power out of the motor is adversely impacted by mechanical inefficiency due to factors such as friction, gear losses, wind resistance (cooling fans, boundary layer friction, etc.) For purposes of this illustration, these losses are considered to be substantially small to non-existent.
When switch 150 is closed, a circuit is completed that allows current, to flow. The following voltages in expressions (1) to (3) are presented relative to ground:Vbat=Vev−(current*Rb)  (1)Vmotor=Vbat  (2)Vemf=Vmotor−(current*Rm)  (3)Assuming negligible mechanical losses, power out of the motor (WO, watts out) is described by expression (4):WO=current*Vemf  (4)At light motor loads, current is low and watts out (WO) is low. At higher motor loads, current is high and WO is high. At the highest motor loads, WO falls from the maximum and significant energy is lost in Rb and Rm. The power lost in Rb and Rm may be calculated as shown in expressions (5) and (6):Power lost in Rb=current2*value of Rb(I2Rb)  (5)Power lost in Rm=current2*value of Rm(I2Rm)  (6)
Table 1 provides an example of losses in power in a DC motor system comprised of an 18 volt battery with 150 milliohm impedance and a DC motor with 60 milliohm impedance.
TABLE 1Power losses in DC motor systempower lostVbat &power lostpower out ofcurrentin RbVmotorin RmotorVemfmotor (WO)(amps)(watts)(volts)(watts)(volts)(watts)0018018054172178510151761615915341614152232060152414276259414381331930135145412351351841374113734024012961038445304111229385503751115083755545410182635560540921653246563482544283707358294323175844733821698096063841968510845434013
Referring to Table 1, a maximum power out value of 385 Watts occurs at 45 amps. As current is increased beyond 45 amps, the motor watts out actually falls as more and more energy is converted to heat in Rb and Rm. This peak power out of the motor of 385 watts that occurs at 45 amps is defined as max watts out of the motor, or MWO.
An understanding of MWO having been described, a comparison of the power-to-weight ratios of a corded power tool with the power-to-weight of a conventional cordless power tool system illustrates a dramatic contrast in performance. In an example, a conventional corded hand-held power drill may produce power (MWO) from a universal motor in the range of between 520-600 Watts. The total weight of the drill is approximately 3.3 to 4.3 lbs. This results in a power-to-weight ratio from about 140 Watts/lb to 158 Watts/lb. In comparison, a conventional 12 volt cordless power tool system, such as a cordless drill with attached NiCd battery pack, produces a MWO from the motor at about 225 Watts at a total tool+pack weight of 4.9 lbs (tool weight of about 3.4 lbs; 12V NiCd battery pack weight of about 1.5 lbs). This results in a power-to-weight ratio of about 46 W/lb.
At least two reasons may explain the substantial differences in the power-to-weight ratios between corded power tools and cordless power tool systems. First, the power source (alternating current) in a corded tool does not contribute to the overall weight of the system since it is not a constituent element of the tool. In contrast, the power source in a cordless tool, the battery pack, is one of the largest contributors of weight therein. Second, the motor in a corded power tool is a universal motor operating off alternating current whose field magnetics are generated by relatively lightweight wiring in the armature windings. Cordless systems, in contrast, typically use DC motors with permanent magnet motors that are comparatively heavier than universal motors because the field magnetics are generated by permanent magnets instead of the lighter wires.
Increasing the power and size of conventional battery packs in a cordless power tool is not a realistic solution for narrowing the gap in power-to-weight ratios between corded power tools and cordless power tool systems. Depending on the anticipated use of the cordless tool, the weight of conventional battery packs required to produce power levels in line with corresponding corded tools render the cordless systems ergonomically inefficient, as the cordless tool becomes too heavy to use, especially over extended periods of time.
Conventional battery packs for cordless power tools above 12 volts typically include battery packs having a nickel cadmium (“NiCd”) or nickel metal hydride (“NiMH”) cell chemistry. As the power output requirements have increased, so has pack weight. A conventional NiCd battery pack capable of delivering 12 volts (or 225 MWO) of power in a cordless tool such as the Heavy-Duty ⅜″ 12V Cordless Compact Drill by DEWALT weighs approximately 1.5 lbs, where the weight of the tool and pack is about 4.9 lbs. Thus, almost one-third (31%) of the overall weight of the primarily single-hand use 12V power drill is attributable to the battery pack.
A conventional 18V NiCd battery pack weighs about 2.4 pounds (2.36 lbs.), representing about 46% of the weight of a power tool such as a Heavy Duty, ½″, 18V Cordless Drill by DEWALT (total system weight (pack+tool) about 5.2 pounds, various 18V models). A conventional 24V NiCd pack weighs about 3.3 pounds, representing about 38% of the total weight of two-handed power tool such as a Heavy-Duty, ½″, 24V Cordless Hammerdrill by DEWALT, Model DW006 (total system weight of about 8.7 pounds).
Thus, increasing the overall weight of the cordless power tool by adding battery packs capable of supplying higher power levels also may negatively influence the ergonomic aspects of the tool by increasing its overall weight beyond acceptable levels. With NiCd and NiMH power sources, higher power means substantially heavier battery packs. The corresponding increases in overall weight of the cordless tool make the tool more difficult to manipulate and/or use over extended periods. For example, the weight of a 24 volt NiCd battery pack (about 3.3 lbs) represents over a 100 percent increase in weight as compared to the weight of a 12 volt NiCd battery pack (1.5 lbs).
The additional weight associated with heavier battery packs may also adversely affect the overall balance of the cordless tool and its ergonomic qualities. Battery packs are traditionally attached to a cordless drill at the distal end of a grip (such as at the bottom of the tool) or near the rear portion of the tool, such as for a cordless circular saw. As voltages increase and the battery pack becomes heavier, the pack weight is leveraged against the remainder of the cordless tool system, potentially making the tool harder to control and use.